1. Field of the Invention
The present invention relates to an economical process for the removal of components to be separated from technical gases in absorption and desorption processes.
2. Description of the Related Art
Such technical gases are mostly natural gas or synthesis gas, the synthesis gas being generated from fossil raw materials such as crude oil or coals and from biological raw materials. Natural gas and synthesis gas contain useful valuable gases but also interfering components, such as sulphur compounds, in particular sulphur dioxide, carbon dioxide and other components to be separated such as hydrogen cyanide and water vapour. Beside natural gas and synthesis gas, flue gases from an incineration of fossil fuels are also included in the group of technical gases from which interfering components as, for example, carbon dioxide, are removed. The components to be separated may also be useful gases which are to be separated for a specific purpose.
Both physical and chemical absorbents can be used for absorption. Chemically acting absorbents are, for example, aqueous amine solutions, alkali salt solutions, etc. Selexol, propylene carbonate, N-methyl-pyrrolidone, morphysorb, methanol, etc. are physical absorbents.
It is known from prior art to remove components to be separated from technical gases in a circuit by means of absorption and desorption processes. The components to be separated are absorbed in the absorption device by the liquid absorbent. The gas which is insoluble in the solvent leaves the absorption device at the top, whereas the components to be separated remain in dissolved state in the liquid absorbent and leave the absorption device at the bottom. Before the laden solution is fed to the top of the desorption device for desorption, the laden solution is usually pre-heated by heat exchange with the hot, regenerated solution, by which part of the energy required for the desorption in the desorption device is recovered.
By means of a heating agent, a reboiler at the bottom of the desorption device serves to generate steam by partial evaporation of the solvent at the bottom inside the desorption device. Here, the generated steam serves as stripping agent to remove the components to be separated from the laden solution. The laden solution is freed by the stripping agent in countercurrent from the absorbed components to be separated. The stripped components to be separated leave the desorption device at the top, with the steam fraction of the stripping agent being condensed in a head condenser and returned to the desorption device. The regenerated solution which has been freed from the components to be separated leaves the desorption device at the bottom, with the solution usually being cooled after heat exchange has been carried out and returned to the top of the absorption device. This concludes the circuit of the absorption and the desorption process.
In the absorption, which in most cases is carried out at a working pressure of 1 to 100 bar, an absorption temperature of 20° C. to up to 70° C. has proved to be advantageous for removing the components to be separated from the technical gas.
The solution laden with the components to be separated can be regenerated by flashing to a lower pressure and/or stripping, the components to be separated being released again and/or stripped by means of steam. Upon completion of the regeneration process the absorbent can be cooled as required and used again.
The temperature required for desorption in a desorption device is higher than the temperature for absorption by means of the absorbent in an absorption device. The desorption device is usually operated at a temperature of 80° C. to 140° C. and an absolute pressure of 0.2 to up to 3 bar.
In absorption and desorption processes, heat recovery can be achieved in a heat exchanger by heat exchange between absorption solution to be heated and absorption solution to be cooled. This heat exchange serves on the one hand to pre-heat the fluid to be heated in a desirable manner, on the other hand the fluid to be cooled is cooled in a likewise desirable manner, so that the energy required for regeneration and supplied from external sources is significantly reduced.
Even in the case of an ideal heat exchange, in which the temperature approximation between the hot, regenerated solution and the heated laden solution is nearly zero, the absorption and desorption processes still require a lot of external energy for the regeneration of the solvent. For economical reasons, the heat exchanger is usually designed for a minimum temperature approximation of approx. 10 K between the hot, regenerated solution and the heated, laden solution. This results in an increase of the regeneration energy demand to be met by external sources.
EP 1 606 041 B1 discloses a method for the selective removal of sour-gas components from natural gas or synthesis gas, with the sour-gas components being removed selectively within two absorption stages by flashing the laden solution in two stages in a flash vessel to a selected pressure and then introducing it for desorption into the desorption device.
Another method for removing sour gas from natural gas is disclosed in DE 10 2005 030 028 A1 where the pressure of the laden solution between the absorption column and the stripping column is controlled stepwise in a way to ensure that as little additional energy as possible is required.
WO 2010/086039 A1 describes a method and a device for separating carbon dioxide from an exhaust gas of a fossil-fired power plant. The absorption and desorption process connected to the power plant combines the “split feed” and the “lean solvent flash” operating modes, in which a more favourable overall plant efficiency of the power plant process is only achieved if the two process stages are combined. The application of the method according to WO 2010/086039 A1 involves a significantly greater demand for equipment than in prior art, as it would here be necessary to install both a vacuum compression stage and another compression stage.
EP 1 736 231 A1 discloses a method and an apparatus for the removal of carbon dioxide, with several variants aiming at improving the energy efficiency being presented. On account of the thermal connection described, however, it is possible to recover only part of the energy which is supplied to the regeneration device, as most of the energy which is still contained in the exhaust steam from the flash vessel described in the disclosure is not recycled to the regeneration device but removed by an external cooler, thus being lost for the system. To ensure that the heat is transferred in an adequate manner from and to the enriched solution of the flash vessel, there is a significantly greater demand for equipment in form of additional heat exchangers, coolers, etc. A necessary intermediate feed in the absorber, for example, increases the required overall height of the absorber and thus also the cost.
On account of the increasing demand for resources an economical mode of operation in all fields has long become an important basis for the further development. The aim is therefore to provide an efficient and cost-saving process by which the energy demand of the whole plant is reduced.